Modeling of polarized radiances in the visible through IR regions requires consistent models for reflectance and emittance of materials. These models must include common effects such as directionality and spectral shape. We have started from a well validated but non-polarized Bi- Directional Reflectance Distribution Function/Directional Emittance model and have added in the polarization state description via Fresnel scaling. The methodology is discussed along with our approach to dealing with possible inconsistencies. Results are demonstrated within a complete 3D polarized object model.
Supported by the Army Humanitarian Demining MURI, we most recently have focused on determining the unique strengths of passive IR sensing as a function of attribute diversity. Our initial findings identify polarimetric hyperspectral imaging.as a robust means to rapidly survey and detect partially exposed, non-metallic anti-personnel (AP) mines. We are investigating the discrimination gains expected from the combined polarimetric hyperspectral attributes under laboratory and field conditions. A principal components analysis of our earliest data indicates that this combination of attributes is about three times more effective in discriminating AP mines or mine-like materials than conventional hyperspectral sensing. In addition, we have uncovered a distinguishing spectral behavior of the Fresnel reflectance across resonance features that can be measured only by spectrally-resolved polarimetry.
We are applying a novel LWIR (long-wavelength infrared: 8-12 micrometers) hyperspectral polarimetric imager to geophysical ocean surface sensing. The sensor is an embodiment of an invention by Aerodyne Research known
as P-SIM (for Polarimetric Spectral Intensity Modulation). The design enables instantaneous ”snapshot” spectropolarimetric imaging with perfect channel registration and full elliptical polarimetry. The optical polarimetry enables retrieval of the phenomenologically rich capillary wave structure morphology (roughness). The spectrally resolved polarimetry additionally allows disambiguation of downwelling from emitted components. Such data supports improved retrievals of sea surface temperature, emissivity, and surface wind vector. We discuss the sensor design, and the analytic means for the geophysical retrievals.
Over the past year with the support of the Army Humanitarian Demining MURI, Aerodyne has substantially moved forward in developing and demonstrating the value of an affordable and fieldworthy IR polarimetric hyperspectral imager for inclusion in multisensor demining. Such technology promises powerful clutter suppression and enhancement of man made objects, particularly applicable to the reliable detection of scatterable mines, especially plastics, and any UXO that are partially exposed. We have achieved the first 3 steps of a 4 step, controlled-risk program defined as follows: (1) LWIR Spectral Polarimeter to demonstrate the effectiveness of combined polarimetric and hyperspectral discrimination capabilities in observations on static scenes; (2) LWIR Uncooled FPA Imaging Polarimeter to verify the sensitivity of an affordable Uncooled FPA in a broadband configuration against static scenes; (3) Multispectral IMaging Polarimeter to quantify clutter rejection performance improvements to be realized from multispectral imaging polarimetry; and (4) IR Polarimetric Hyperspectral Imager designed with optimal spatial and spectral resolution and sufficient throughput to achieve the reliable performance required in surface mine and UXO detection applications. We present results for Steps 1 and 2, and initial result for Step 3 from the ongoing demonstrations in simulated surface mine detection.
We introduce the method of Polarimetric-Spectral Intensity Modulation (P-SIM) and discuss how it enables a new robust class of hyperspectral polarimetric imaging sensor. P-SIM was invented by one of us and has been submitted for patent. We are presently building a sensor, dubbed the IR Polarimetric HyperSpectral Imager which implements the P-SIM concept. P-SIM employs a novel and robust optical multiplexing scheme that enables simultaneous measurement of spectral and full elliptical polarimetric image content, employing a single focal plane detector and conventional optics, and eliminating moving parts and difficult alignment issues. The technique is equally viable across the visible through long-wave IR bands. The P-SIM concept constitutes a breakthrough for the inclusion of polarimetry in optical hyperspectral imaging. To date, even single-band polarimeter designs for remote sensing are compromised due to their lack of spatio temporal measurement registration, inapplicability to marginally resolved scene elements, costly optical configurations, or polarimetric ambiguity from too few 'channels'. Polarimetric imaging from a moving platform or against moving targets rules out the standard methods of time-sequential polarimetry via rotation of a polarizer or waveplate. P-SIM eliminates these limitations while additionally extending polarimetry into the spectral imaging domain.
The augmentation of passive IR conventional and hyperspectral imaging sensors with polarimetric capability offers enhanced discrimination of man-made and geophysical targets, along with inference of surface shape and orientation. In our efforts to size the design of IR polarimetric hyperspectral imagers to various remote discrimination applications, we have ascertained critical relationships between polarimetric SNR and pixel sizing. This relationship pertains primarily to realms wherein the objects to be sensed will be marginally resolved spatially. The determination of such application- specific relationships is key to the design of effective polarimetric sensors. To quantify this key trade-off relationship, we have employed the latest developmental version of SPIRITS, a detailed physics-based signature code which accounts for the various geometric, environmental illumination, and propagation effects. For complex target shapes, detailed accounting for such effects is especially crucial to accurate prediction of polarimetric signatures, and thus precludes hand calculation for all but simple uniformly planar objects. Key to accurate polarimetric attribute prediction is our augmentation of the Sandford-Robertson BRDF model to a Mueller/Stokes formalism that encompasses representation of fully general elliptically polarized reflections and linearly polarized thermal emissions in strict compliance with Kirchoff's Law. We discuss details of the polarimetric augmentation of the BRDF and present polarimetric discriminability-resolution trade-off results for various viewing aspects against a ground vehicle viewed from overhead.
Several years of effort in IR polarimetry have brought us convincing evidence of its effectiveness in differentiating man made objects from natural backgrounds. Adding modern focal plane array (FPA) technology (either cooled or uncooled) makes it possible to combine the benefits of polarimetry with the power of hyperspectral imaging. Aerodyne Research is embarked on a stepwise, controlled-risk development program with the objective of fielding an innovative and affordable hyperspectral imaging IR polarimeter. Proof-of-concept demonstrations are conducted for each significant technology increment as part of the prototype development effort. These steps, two demonstrated and two yet to be demonstrated, are: (1) LWIR (non-imaging) Spectral Polarimeter to demonstrate the effectiveness of combined polarimetric and hyperspectral discriminating capabilities in observations on static scenes; (2) LWIR Uncooled FPA Imaging (broadband) Polarimeter to test the sensitivity of an affordable Uncooled FPA in a broadband configuration against static scenes; (3) Multispectral Imaging Polarimeter to quantify clutter rejection performance improvements to be realized in multispectral polarimetry; and (4) Hyperspectral Imaging IR Polarimeter designed with optimal spatial and spectral resolution and sufficient throughput to achieve the reliable performance required in surface mine and UXO detection applications. Results from the ongoing proof-of- concept demonstrations in simulated surface mine detection will be presented.
KEYWORDS: Polarization, Land mines, Polarimetry, Infrared signatures, Mining, Signal to noise ratio, Coating, Solar radiation models, Infrared imaging, Reflectivity
The passive multispectral IR polarization signature attributes of mines and background are observable to an appropriately designed detection system. The processes that create signature polarization are spectrally dependent. At shorter wavelengths, reflected solar radiation produces polarization which is perpendicular to the plane of incidence. At long wavelengths, the reflected sunlight is relatively weak and polarization of thermal emissions, which are parallel to the plane of incedence, may dominate. A multispectral polarimetric imaging system could measure a scene's percent and angle of polarization attributes in different spectral regimes. These images can be spatially compared to reveal the presence of manmade polarizing features such as the exposed surfaces of mines or anomalous perturbations to normal background. This information would be processed by suitable discrimination algorithms which might cross-correlate the spatial polarimetric and spectral channels. Aerodyne Research, Inc. and Boeing Defense and Space Group of Seattle, WA have investigated the feasibility of employing passive IR multispectral polarimetry to locate and identify land mines. The results of this investigation, which used a combination of model- based analysis and field measurements, are reported.
Aerodyne has recently developed an IRST engagement model under contract for Lockheed Aeronautical Systems Company. The model's purpose is to simulate the performance of an IRST system in long-range air-to-air detection and tracking engagements. The hallmark of the model is its end-to-end first-principles modeling of all major elements which determine specific performance. The target aircraft IR signature, and atmospheric cloud and sky background, and associated atmospheric effects are modeled at high fidelity, thereby producing an input image matched to the specific IRST under study. A detailed deterministic model of the IRST accounts for optical and sensor effects, signal processing, and track association typical of first-generation IRSTs. These model elements are coupled together along with a dynamic target and observer [IRST] trajectories model so that an analyst can specify air-to-air engagements at various velocities, ranges, and viewing angles. The analyst can study the effects of varying IRST algorithms, sensor characteristics, optical bandpass, cloud background levels, atmospheric effects, and target performance characteristics as well as varying the target aircraft itself. This computer model was designed for portability and growth.
KEYWORDS: Infrared search and track, Clouds, Sensors, Performance modeling, Systems modeling, Atmospheric modeling, Signal processing, Data modeling, Data processing, Target detection
Aerodyne has recently developed an IRST Engagement Model under contract for Lockheed Aeronautical Systems Company (LASC). The model's purpose is to simulate the performance of an IRST system in long-range air-to-air detection and tracking engagements. The hallmark of the model is its end-to-end first- principles modeling of all major elements which determine specific performance. The target aircraft IR signature, the atmospheric cloud and sky background, and associated atmospheric effects are modeled at high fidelity, thereby producing an input image matched to the specific IRST under study. A detailed deterministic model of the IRST accounts for optical and sensor effects, signal processing, and track association typical of first-generation IRSTs. These model elements are coupled together along with a dynamic target and observer (IRST) trajectories model so that an analyst can specify air-to- air engagements at various velocities, ranges, and viewing angles. The analyst can study the effects of varying IRST algorithms, sensor characteristics, optical bandpass, cloud background levels, atmospheric effects, and target performance characteristics as well as varying the target aircraft itself. This computer model was designed for portability and growth.
Aerodyne has recently developed an 'IRST Engagement Model' under contract for Lockheed Aeronautical Systems Company (LASC). The model's purpose is to simulate the performance of an IRST system in long-range air-to-air detection and tracking engagements. The hallmark of the model is its end-to-end first-principles modeling of all major elements which determine specific performance. The target aircraft IR signature, the atmospheric cloud and sky background, and associated atmospheric effects are modeled at high fidelity, thereby producing an input image matched to the specific IRST under study. A detailed deterministic model of the IRST accounts for optical and sensor effects, signal processing, and track association typical of first-generation IRSTs. These model elements are coupled together along with a dynamic target and observer (IRST) trajectories model so that an analyst can specify air-to-air engagements at various velocities, ranges, and viewing angles. The analyst can study the effects of varying IRST algorithms, sensor characteristics, optical bandpass, cloud background levels, atmospheric effects, and target performance characteristics as well as varying the target aircraft itself. This computer model was designed for portability and growth.
Synthetic scene generation models depend on the repeated evaluation or iteration of often computationally expensive functions to create 'texture' or structure in a modeled natural scene. The computational burden of 'texture' function evaluation is dependent upon the spatial frequency response characteristics of the optical system through which the modeled scene is to be 'propagated.' All real 'cameras' utilize detector elements of finite size to transduce the resulting MXN pixels of the image. Two general situations arise in modeling the effects of cameras (sensors) on the imaged scene. The first is the 'oversampled' case where the Nyquist frequency of the detector spacing exceeds the cutoff of the optics transfer (aperture) function. The 'undersampled' case is when such Nyquist frequency is below the aperture cutoff spatial frequency, which results in aliasing. The application of brute force in the oversampled case results in MXN scene 'texture' function evaluations, while for the undersampled case, (mM)X(nN) (m, n > 1) points are required. Several computationally efficient methods significantly reduce the stated brute force computational burden for these two cases. This paper discusses three such methods. For the oversampled case, a correct and efficient method is 2D sample 'interpolation,' in the multirate digital signal processing sense. This method expressly avoids signal aliasing caused by simpler but inappropriate bilinear interpolation of the sparser set of (mM)+(nN) (m, n fractions < 1) scene samples onto the MXN imaged scene. The second and third techniques discussed are applicable to the undersampled case. Each relies upon MXN scene 'texture' function evaluations. The second technique extrapolates the frequency spectrum of the MXN grid with a synthetic spectrum beyond Nyquist which follows the 1/f(beta ) decrease in power typical of natural (fractal) textures. The third technique, 'fractal interpolation,' operates in the spatial domain where spatial detail, generated from the computed ('texture' function) MXN grid onto a larger (nM)X(nN) grid, is synthesized at the same local fractal dimension as that of the MXN 'undersampled' data. In both cases, the synthesized frequencies above the camera Nyquist are 'folded back' in the spectral domain to approximate the aliasing of spatial frequencies into the transduced image.
The optical performance of near-horizon viewing infrared airborne sensors is affected by atmospheric variations along the line of sight, which are associated with (a) ambient atmospheric vertical structure and turbulence, and (b) the aircraft boundary layer. We have modeled systematic (non-fluctuating) errors and random variations in line-of-sight (LOS) angle and target intensity for several MWIR and LWIR wavelengths as a function of LOS elevation angle from a high altitude aircraft-borne sensor. A range of atmospheric temperature, density, and structure models was used to investigate the sensitivity of target scintillation, jitter angle, and image spread effects to altitude.
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